1. Field of the Invention
The present invention relates to packet-based communication technology, and more particularly to recovery of a received signal at a receiver in a packet-based communication system.
2. Description of the Related Art
IEEE 802.11 is a well know packet-based wireless communication standard using OFDM (Orthogonal Frequency Division Multiplexing) modulation technique and is now adopted as the standard in wireless LANs (Wireless Local Area Network).
The IEEE 802.11 specification defines requirements for a physical (PHY) layer and a medium access control (MAC) layer, which is one of the two sub-layers in a data link layer according to the OSI (Open System Interconnection) network model. Commonly-seen IEEE 802.11a, 802.11b and 802.11g are different PHY layer standards and they all follow the same MAC layer standard.
FIG. 1 shows a frame format in the PHY layer of the IEEE 802.11a standard. As shown, a packet frame is divided into three portions, a preamble portion 10, a header portion 12 and a data portion 14. The preamble portion 10, which is also referred to as a training sequence, consists of a short preamble field with 10 short symbols and a long preamble field with two long symbols. In a WLAN receiver, inherent problems, such as DC offset and AGC, must be solved. In a high speed communication, the transmitted and corresponding received signals are often different from each other, and therefore, timing synchronization and frequency offset estimation must be performed. In addition, the transmission channel is unknown at the receiver and thus must be estimated. These are achieved based on the training sequence in the preamble field at the beginning of each frame format.
The header portion 12 is used for carrying information about the coding rate and the length of the packet.
The data portion 14 includes the PSDU field (i.e., packet data), the tail bits (six “0's”) and the pad bits.
FIG. 2 shows a general frame format in the MAC layer of the IEEE 802.11 standard. When the packet data is decoded in the PHY layer and delivered to the MAC layer, it will be further identified whether or not the current packet is intended to be transmitted to this receiver. This is performed by examining whether or not the destination address (DA) in the field “Add 1” in FIG. 2 matches an identity address of the receiver (for example, the MAC Address on a network interface card). In addition, a CRC checksum calculation is performed to verify the received packet data, that is, to detect transmission errors, by using the 32-bit CRC code contained in the field “FCS (Frame Check Sequence)”.
In the above exemplified IEEE 802.11a specification, the preamble portion 10 is very short, with a very limited training duration of only 16 μs for the receiver to retrieve all the required information in order to decode data correctly. This implies a complicated algorithm and a high cost to implement the estimation circuit.
Therefore, there is a need for development of a receiver with a simpler algorithm for retrieving the required information while allowing the estimated data, such as DC offset estimation, frequency offset estimation and channel estimation, in the receiver to well match with the transmitted data to improve the receiver performance.